I found the explanations in this book to be very clear, giving enough detail for a good head start into Linux internals. The book briefly explains the OS concepts, such as semaphores, virtual memory, etc., followed by an overview of how each is implemented in Linux, and code snippets.

Most of the code snippets are simplified for readability, which I found useful because the hacks can be distracting (scary, too) for a beginner. Detailed and up-to-date information can best be obtained from source code itself.

The book assumes some familiarity with Unix concepts, as it mentions such buzzwords as POSIX, BSD, and SVR4 in the context of the discussions, but one could safely ignore them, and just concentrate on the Linux part.

The book briefly covers adding new system calls, compilinag and debugging the kernel, and even shows how to write a simple device driver - these are hard to find in one place.

Overall, I found this book to be very useful for my self-paced study (the best so far), and I only wish they had a newer edition.